1. Field of the Invention
This invention relates to a rolling or tilting responsive switch mounted on an automatic shutoff valve having an integrated microcomputer and employed in a city gas equipment and a commercial propane gas equipment or mounted on control devices of oil space heaters, gas burning appliances and electrical equipment, for detecting oscillations such as an earthquake to thereby supply a detection signal to the automatic shutoff valve or the control devices.
2. Description of the Prior Art
The prior art has proposed various types of seismosensitive device. Japanese Laid-open Patent Application (Kokai) No. 63-29286 (1988), which will be referred to as "first reference," discloses a seismosensitive device wherein a seismosensitive ball drives a first contact in response to an earthquake of a predetermined seismic intensity to cause the same to be engaged with a second contact. The seismosensitive ball is designed to automatically maintain its horizontal state in a casing. Japanese Laid-open Patent Application (Kokai) No. 2-186224 (1990), which will be referred to as "second reference," discloses a seismosensitive device wherein a seismosensitive ball rolls in a receptacle such that a movable contact suspended from over the ball is engaged with and disengaged from a fixed contact disposed in an inner casing. The receptacle is suspended in an outer casing with a liquid filled therein. Japanese Laid-open Patent Application (Kokai) No. 64-79624 (1989), which will be referred to as "third reference," discloses a seismosensitive device wherein a mercury globule is enclosed in a metal receptacle.
The seismosensitive devices have recently been mounted on automatic shutoff valves having an integrated microcomputer and employed in a city gas equipment and commercial propane gas equipment. Oscillations due to an earthquake or the like are detected by the seismosensitive device, which thereby generates a detection signal. The signal is supplied to the automatic shutoff valve so that necessary countermeasures are taken. In these valves, the oscillations due to the earthquake need to be distinguished from an oscillation caused by collision of a flying object with the valve and an artificial noise caused by driving of a dump truck or the like, civil engineering works and the like. For this purpose, the seismosensitive device is required to have predetermined operating characteristics in a frequency band of the oscillations due to the earthquake and different operating characteristics in the other frequency band. The above-mentioned first reference discloses nothing about this requirement. More specifically, in the disclosed seismosensitive device, the movable contact is held between the fixed contact and the ball serving only as the driving source of the movable contact when the movable contact is engaged with the fixed contact. Actually, however, the substantial collision of the ball with the fixed contact, both members being rigid bodies, takes place and a resultant repulsion causes the ball and the movable contact to instantaneously depart from the fixed contact. This construction cannot ensure a desirably long contact time period and accordingly, the duration of an "on" signal cannot be rendered long enough when the seismosensitive device serves as a switch. Thus, the oscillations due to the earthquake cannot be distinguished from that due to the artificial noise or electrical noise when the device disclosed in this reference is employed for detecting the oscillations due to the earthquake.
The movable contact is suspended with a point of support slightly higher than the center of gravity of the seismosensitive device disclosed in the above-mentioned second reference, which increases the number of parts and complicates the construction of the seismosensitive device. Consequently, it is difficult to assemble and miniaturize the seismosensitive device. Furthermore, this seismo-sensitive device cannot provide a long contact time period as in the device of the first reference. The second reference discloses that the repulsion due to the collision of the movable contact with the fixed contact is absorbed by resilient members so that the contact is prevented from being instantaneous. However, this further increases the number of parts and complicates the construction. Although this reference also discloses that the seismosensitive ball can be rendered small since the movable contact is moved with the ball, a friction between the ball and the movable contact prevents the miniaturization of the ball. Additionally, the receptacle enclosing the ball is formed from an insulating material. Where the insulating material is a synthetic resin, organic contaminants which cause failure in electrical conduction between the contacts are apt to be produced. The production cost of the seismosensitive device is increased where the insulating material is a glass or ceramic. Furthermore, the rolling of the ball or the collision thereof with the movable contact deforms, particularly, projected or corner portions of the receptacle when the receptacle is formed from the synthetic resin. Consequently, initial operating characteristics cannot be achieved after some period of service.
The seismosensitive device employing the mercury as disclosed in the third reference is a high-performance switch having characteristics matching with control by means of a microcomputer and providing a stable performance for a long period of time. However, a seismosensitive device wherein the mercury is not employed has recently been desired from a view of protection against environmental contamination.
In view of the foregoing, a small-sized, rigid, cost-effective seismosensitive device suitable for the mass production has been desired. The mercury should not be employed in the seismosensitive device and yet, it should have the same operating characteristics as in those of the seismosensitive device employing the mercury. In this respect, however, the construction of the seismosensitive device employing the mercury globule cannot automatically be applied to the seismosensitive device wherein the solid conductive ball is employed, since the mercury globule is liquid.